Flame photometer using vibrating slit monochromator

ABSTRACT

A flame photometer for use in monitoring processes wherein the fuel gas and/or sample produce a high background emission spectrum and a monochromator for detecting the signal of interest with the background being removed by filtering the signal with a vibrating slit in the monochromator. The system also includes a means for automatic standardization of the recorded signal.

July 3, 1973 United States Patent 1191 Jobe FLAME PHOTOMETER USING VIBRATING SLIT MONOCHROMATOR OTHER PUBLICATIONS The IL Flame Photometer 143; Instrumentation Laboratory Inc. Wate rtown, Mass.

The Application of Process Stream Analysers in Air Apr. 15, 1971 Separation Plants; Fraade; Process Control & Automation; Feb 1965; Pg- 62-65 211 Appl. No.: 134,173

ABSTRACT Primary Examiner-Ronald L. Wibert Assistant-ExaminerV. P. McGraw AttorneyTheodore E. Bieber and Harold L. Denkler A flame photometer for use in monitoring processes W84%2 I 6W 1 swoon 3 1 1 2 1 m w 6 4 1 5 5 Ha m m U G W 1 5 "7 3 WW9 m WW6 ""9 uu U "U U u me n .r. u a u h L .f. C [0 s cm k U .mF N UN 5 55 1' References Cited I wherein the fuel gas and/or sample produce a high UNITED STATES PATENTS background emission spectrum and a monochromator for detecting the signal of interest with the background being removed by filtering the signal with a vibrating slit in the monochromator. The system also includes a meansfor automatic standardization of the recorded 9 Claims, 5 Drawing Figures 777S746 889 829 666 6/6 555 565 333 353 .6 3 nn 5 m m 1 ma m m m m m .d m e u r. r m e lt g ".mh Y M .1 6 0 MRKMRHL 9 9402 6776677 9999999 1111111 3259922 476323 66622 3 05009656 3676494 3333333 1 MONOCHROMATOR 20 30 JP/HER PRESSURE REGULATOR FLOW 1 1 mum/110145 l 5 21 1 FUE'L GAS //v 47 SAMPLE //v 1 PUMP PAIENIEML 3 ma sum '8 If 3 INC/DENT RADIATION as MONOCHROMATOR 87 79 N 1 PHOTO- Ex/T MULT/PL/ER SL/T DRIVER 51 [F/LTER! ELECTRON/C ADJUSTABLE SWITCH PHASE L RECORDER sH/FTER 84 T 82 j FREOuENcv DOUBLER /83 96 95 FULL SPAN REFERENCE COMPARATOR/ 100 RESET AND AcT/vATE SIGNALS RAMP VOLTAGE gggfig GENERATOR 92 A TA/D cONvERTER MULT/PL/ER 91 94 j /9(3SUMM/NG AMPLIFIER FIG. 2

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IN VE N TOR FLAME PHOTOMETER USING VIBRATING SLIT MONOCHROMATOR BACKGROUND OF THE INVENTION The present invention relates to flame photometers and particularly to process-type flame photometers which are capable of detecting small quantities of metallic elements in liquids such as water or hydrocarbon streams. Most specifications for hydrocarbon fuels place extremely small limits on certain metallic elements. For example, in the case of turbine fuel, alkali metals are limited to a range of less than 0.05 parts per million. Similarly, in unleaded gasoline, lead content may be limited to a few parts per million.

Monochromator detectors have been used in combinations with flame photometers for detecting low levels of metals in various liquids. The flame photometer consists of a flame or burner assembly in which a sample of the liquid containing the metal is burnt with a fuel gas and air supply. Each metallic element which is sufficiently excited in the flame will emit light of a particular wavelength known as a spectral line characteristic of the metallic element. Such a spectral line is the light of interest. The monochromator is disposed to intercept a portion of the light from the flame and by means of a prism or other suitable device to disperse the light from the flame such that a spectrum is produced. By means of a suitably placed exit slit or by means of changing the position of the prism or other dispersive device relative to the exit slit a spectral band covering a narrow wavelength interval containing. the spectral line is caused to exit from the monochromator. Such a spectral band in some cases may contain unwanted background light from the flame.

In order to discriminate against stray light not orginating in the flame or detector noise, it has been the practice in the past to use some means to periodically interrupt the spectral band containing the light of interest, a technique commonly referred to as chopping. For example, rotating mirrors or prisms have been used to chop the light so that the detector, which is normally a photomultiplier tube, produces an alternating signal from the chopped spectral band. The selected spectral band, of course, is chosen to correspond to the particular metallic element in which one is interested. The use of various chopping devices is not completely satisfactory since they do not discriminate against the flame background light in the spectral band. In order to partially overcome this defect, it has been the practice to use low background producing gases for the fuel gas, for example hydrogen. Another technique used with prior art devices requires introducing a sample free of unknown metallic element followed by a sample con taining the unknown element. The former sample de termines the background signal which must then be subtracted from the latter sample signal by some man-' ual action. I

While the use of low background noise gases eliminates much of the problem, the gases are expensive, dangerous and require storage in a gaseous form. This limits the usefulness of the flame photometer as a process control device since it is normally impossible to store sufficient gas such as hydrogen to permit continuous operation of the device at a plant location. It is also sometimes desirable to burn the sample stream directly, without a fuel gas, causing a background produced by the sample combustion.

Further problems arise with prior art devices in their inability to correlate the recorded signal with the actual quantity of the metallic element present in the process stream. Thus there is a need for some means for continuously correlating the recorded signal with known standards so that one can determine tthe actual amount of the metallic element in the process stream.

BRIEF DESCRIPTION OF THE INVENTION The present invention solves the above problems by providing a monochromator which can effectively detect the signal of interest of a flame photometer operating on conventional fuel gas and air or combustible sample liquid and air. Thus, the equipment can operate on propane gas and a compressed air supply that are normally available in plants where process streams are analyzed for metallic elements. This permits the equipment to be operated continuously as a process control device. The unwanted effect caused by background light is eliminated from the signal by use of a vibrating slit for the exit of the monochromator. In one mode of operation the vibrating slit is made to pass the light of 1 interest emitted by the metallic element plus the background light in one extreme position of its cycle, while in the opposite extreme position of its cycle only the background light is passed. Provided the background light is reasonably constant over the spectral range scanned, the resulting alternating signal from the detector will be proportional to the difference in intensity of the light intercepted at the cycle extremes. The resulting alternating signal will therefore only be due to the light of interest since the background light contributes only a steady or constant signal.

A second mode of operation may be used to reject background contributions when the background is not constant but has a definite slope with respect to wavelength. The exit slit is caused to scan completely across the spectral band containing the light from the metallic element. Thus at one extreme of the scanning cycle background light is passed and at the opposite extreme only background light is passed. Any difference in background intensity will result in a signal at the fundamental scanning frequency. However, the spectral light from the element of interest will tend to be passed midway between the cycle extremes and will produce a signal of twice the fundamental scanning frequency. The signal amplifier may thus be tuned to twice the fundamental frequency and will thus not pass the background signal.

The invention also includes a means for automatically correlating or adjusting the recording device so that it will record a signal which reads directly in concentration units of the metallic element present in the process stream. Normally a continuous chart recorder is used as a recording device to produce a continuous record which may be retained. The adjusting circuit comprises a comparing circuit wherein a signal representing a known quantity of a metallic element is compared with a signal representing a known displacement of the pen of the chart recorder; for example, full scale deflection of the pen. Any difference between the two signals is then used to adjust the chart recorder so that the pen is deflected full scale in response to the sample having known concentrations of the metallic element. The circuit also includes means by which a sample containing a known quantity of metallic element can be injected into the flame photometer at preset intervals.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be more easily understood from the following detailed description when taken. in conjunction with the attached drawings in which:

FIG. 1 is a drawing of an instrument constructed according to this invention and shown in block diagram form;

FIG. 2 is a circuit of the instrument shown in FIG. 1, also in block diagram form, and

FIGS. 3b and 3c are a series of approximate wave forms corresponding to signals that are produced in the instrument of this invention and are compared to conventional signals shown in FIG. 3a.

DESCRIPTION OF A PREFERRED EMBODIMENT Referring now to FIG. 1, represents the burner assembly of the flame photometer having a flame jet 11 with a flame arrester 12 disposed therein. The heat from the flame jet exhausts through the chimney 17 which is provided with a flame arrester 13 at the top. The use of the flame arrester l3 and other explosionproofing devices is only required when the flame photometer is used in a dangerous atmosphere such as monitoring a process stream at a refinery location. The burner assembly is provided with an igniter 14 which is powered from a suitable power supply 15.

The jet is supplied with fuel gas from a source by a line 24 having a regulator 21 and a flow indicator 22. As explained, the fuel gas may be any conventional combustible hydrocarbon stored as a liquid or as a compressed gas such as propane, butane or a mixture thereof, or may even be absent when a combustible sample itself is used for fuel. A solenoid operated stop valve 23 is provided on the discharge side of the flow indicator to provide a means for shutting down the device in case of unsafe conditions. The solenoid valve 23 is coupled to a mixing and liquid separating portion 16 of the device by means of the line 24. The air for supporting both the combustion of the flame jet 11 and for purging the burner assembly is supplied from a compressed air source through a pressure regulator 31.

The air for purging the burner assembly passes through a flow regulator 32 which is coupled by a line 33 to the burner assembly. The combustion supporting air passes through a second flow regulator 34 and a flow indicator 35 and is coupled by a line 36 to an atomizing section 37. The atomizing section is designed to atomize the liquid sample in air and supply the aerosol mixture to the flame jet while any liquid remaining is drained off through the filters 51 and 52 to a drain 53.

The sample liquid is supplied from the source 40 to a four-way valve 41 by means of a pump 47. The fourway valve is disposed to supply the sample to the atomizer section 37 through a flow indicator 42 or through a separate line 43 to the drain line 53. This last path to the drain line is used when a standard sample is supplied to the atomizer or it is necessary to shut down the device due to a malfunction. Also supplied to the atomizer section is a standard sample contained in a tank 48 which is coupled by a line 44 to the four-way valve 41. The sample tank 48 is pressurized by the compressed air from the'source 30 through a line 45.

The light emitted by the flame jet 1 l is focused by the lens 46 and enters a monochromator 60 which is described more fully below. A light detecting means, for example a photomultiplier tube 61, is disposed at the exit end of the monochromator to intercept or monitor the particular spectral band of light of interest.

The photomultiplier tube is coupled to an amplifier 62 which is preferably a tuned amplifier and includes means for driving the vibrating exit slit of the monochromator as more fully described below. The amplifier in turn is coupled to an automatic gain adjusting amplifier 63 which is used to position the pen of the chart recorder 64 to adjust its output in relation to the signal produced by a standard sample as described below.

The discharge gas temperature of the flame jet is measured by a temperature measuring means 65 which may be thermcouple or thermistor. The temperature measuring means is coupled to a temperature sensing circuit 66 whose output is used to operate the solenoid valve 23 or to remove power from the sample pump 47. Thus if the gas temperature from the flame jet exceeds predetermined limits, the temperature sensing circuit can close the solenoid 23 and remove power from the sample pump 47 causing a shut off in the supply of fuel to the flame. Also shown in FIG. 1 are power supply 70 for the photomultiplier tube and a timer relay circuit 71 for controlling the position of the four-way valve to supply either the standard sample to the flame jet or the process stream sample. The timer relay circuit also controls the operation of the autogain ranging amplifier 63 so that it either records the signal from the monochromator or resets the chart recorder in response to the signal representing the standard sample.

Referring now to FIG. 2 there is shown in block diagram form the circuits for amplifying the monochromator signal as well as for adjusting the chart recorder so that it records directly in concentration units of a particular metallic element present in the process stream. The monochromator 60 is provided with a fixed entrance slit 85 and a vibrating exit slit 79. The exit slit is driven by a driving circuit 72 in a direction to scan across the particular wave band of interest. For example, if one desires to detect the presence of sodium in turbine fuel the monochromator is adjusted so that the exit slit vibrates in the region of approximately 5,890 angstroms. Further the entrance slit and exit slits should have approximately the same width for best results. The alternating signal from the photomultiplier tube or other suitable detector is supplied to a tuned amplifier 73 which is provided with two filter circuits I 74 and 75 and a switch means 76 for connecting one of the filter circuits in the feedback circuit of the amplifier. The filter circuit 74 is designed to provide maximum gain at the frequency of the slit driving circuit 72 and filter circuit 75 at twice this frequency. The signal from the tuned amplifier 73 is supplied to an amplifier 77 which is disposed tooperate as a synchronous rectifying circuit with resistances 86, 87 and 88 chosen to provide approximately unity gain. The amplifier 77 also receives a signal representing the frequency of the exit slit driving circuit 72 or twicethis frequency. The amplifier 77 receives such signals through an adjustable phase shifter 81, electronic switch 82, a frequency doubler 83 and switch means 84. Thus the phase circuit can supply a signal having the frequency of the driving circuit or double this frequency. The switch 84 is used to switch between the driving frequency or twice this frequency and operates in conjunction with switch 76 in the disposition shown. The signal from the amplifier is passed through a filter '78 of adjustable time constant and sent to the multiplier 91 through lead 90.

The automatic gain adjusting of the signal to the recorder 64 is provided by a circuit including a comparator circuit 95. The comparator circuit is supplied with a fixed reference signal 96 which is equivalent to a fullspan or other fixed displacement of the recording pen of the recorder. The multiplying circuit 91 also receives a signal from a digital storage unit 92. The multiplier multiplies the two signals together and supplies the product to one input, a summing amplifier 93, whose output is supplied to the comparator circuit 95. The second input to the summing amplifier 93 is grounded at 94. In the configuration shown, the output signal of the summing amplifier will be equal to the product composed of the monochromator signal on lead 90 multiplied by the signal from the digital storage unit 92. A reset signal is applied at lead 98 which causes the outputs of the ramp voltage generator analog to digital circuit 97 and the digital storage 92 to be reset to zero. This signal is followed by an activate signal on lead 98 which causes the ramp voltage generator 97 to digitally generate an increasing ramp voltage. This ramp voltage is fed to digital storage 92 through lead 99 and finally to the multiplier 91. When the output of amplifier 93 equals the reference voltage 96 the comparator 95 gives a zero voltage output on lead 89 which inhibits further ramp voltage generation and causes a digital storage of the ramp voltage that exists at the time of inhibition. Such a ramp voltage remains constant and controls the overall gain of the circuit existing between leads 90 and 100 until such time as reset and actuate signals are again supplied on lead 98. The output of amplifier 93 is supplied to a chart recorder 64 for continuous monitoring of standard and unknown samples.

Thus the circuit shown at the bottom of FIG. 2 can be actuated at preset times by suitable switches included in the relay circuit 71 which provide reset and actuate signals whenever a standard sample is supplied to the flame jet of the burner assembly as described above. Such a circuit therefore automatically changes its gain to compensate for sensitivity changes in the flame photometer as determined by the response to a standard sample.

OPERATION Referring now to FIG. 3, there is shown schematically spectral light produced in the exit slit plane by a monochromator that intercepts the light transmitted from a flame photometer operated on a highspectral background sample or fuel gas such as butane or propane. FIG. 3A represents the spectral region 101 which would be passed by a fixed exit slit. A light chopping device such as a rotating prism or a rotating perforated disc would produce an alternating wave form similar to that shown in 102. It is seen that the majority of the signal may be composed of the background level and only a small portion of the signal may relate to the light of interest, i.e., that due to the spectral line only. As explained above, if a hydrocarbon gas such as propane or butane is used as a fuel gas or if a sample is burned directly there will be produced a considerable background level. In the past this has been avoided to some extent through the use of a low background gas such as hydrogen which produces very little background. The applicants device solves the problem of high background levels as shown by the wave form s shown in FIG. 3B and 3C. In FIG. 313 there is shown the spectral region 103 corresponding to the spectral region scanned by the vibrating exit slit. As shown, the vibrating exit slit passes the background level plus the light of interest in one position and then as it moves to a second position it will pass only the background level light. The resulting alternating signal is effectively the result of subtracting the signal in the second position from the signal in the first position. This alternating signal will have a wave form as shown in 104 whose amplitude will be substantially the signal of interest caused by the spectral line without the background level.

FIG. 3C illustrates the type of spectrum that is obtained when the background shows a definite slope with wave length. In order to eliminate the undesirable effect caused by such slope the vibrating slit may be caused to scan the spectral region shown in 105 which would in turn produce an alternating signal composed of the wave forms shown in 106 and 107. When a spectrum is present having a changing background level and the slit is disposed to scan as in 105 it is necessary to provide the synchronous rectifier with a reference signal of twice the fundamental vibrating frequency and to switch the tuned amplifier to pass twice the vibrating frequency. The scanning of the exit slit completely across the spectral line of interest produces a signal 106 of twice the vibrating or scanning frequency. However, the contribution to the composite alternating signal caused by the background slope is a signal 107 at the fundamental frequency. These signals are separated in the tuned amplifier 73 which is switched to the appropriate filter and hence passes only the signal due to the spectral line which is subsequently rectified properly by the application of twice the fundamental frequency as reference through the switching of 84 to activate 83.

When it is desired to calibrate or adjust the recording span of the recorder 64 it is only necessary to position the four-way valve 41 to inject the standard sample 43 into the atomizer 37. This is done automatically by a timer circuit in 71. The standard sample, of course, contains a known amount of the metallic element that is to be detected; for example, it may contain 10 parts per million of sodium in a conventional turbine fuel. When this sample is detected by a monochromator, the adjusting circuit shown in FIG. 2 can be caused to adjust the span of the recorder to full span. Thus the full span reading of the recorder will be equal to 10 parts per million of sodium in a sample. Under these conditions, once the process stream has been readmitted to the flame photometer, it will be a simple matter to read the recorded signal directly in parts per million of sodium present in the process stream being analyzed. The automatic injection of satndard sample usually lasts less than 5 minutes and may be programmed to occur several times per day.

I claim as my invention:

' 1. An apparatus for detecting the quantity of a metallic element in a process stream, said apparatus comprismg:

a burner assembly including means for supplying a fuel gas, air and a sample of the process stream thereto;

a monochromator disposed to intercept the light emitted by the flame, said monochromator having a narrow fixed entrance opening and means for producing a spectrum of the light passing through the entrance opening;

a narrow exit slit opening disposed to pass a narrow band of light from said spectrum, a drive means coupled to said exit slit opening and disposed to vibrate said exit opening at a predetermined frequency;

light detecting means comprising means for detecting the modulation of the light from said vibrating exit slit at the fundamental frequency of vibration of said slit and at twice the fundamental frequency of said slit and converting a signal representing twice the fundamental frequency to a related electrical signal; and

recording means coupled to said light detecting means to record said electrical signal in relation to time.

2. The apparatus of claim 1, and in addition anautoranging means for adjusting the range of the recording means in response to the introduction into the burner of a sample having known amounts of metallic elements.

3. The apparatus of claim 1 wherein said fuel gas is a hydrocarbon.

4. The apparatus of claim 1 wherein said exit slit opening comprises a vane member mounted on said drive means for movement in a plane disposed at right angles to the beam of light from the spectrum producing means, said vane having a narrow elongated slit formed therein for passing said narrow band of light.

5. The apparatus of claim 2 wherein said autoranging means comprises circuit means for injecting into the burner a standard sample having a known composition and a means for adjusting the recorder to a predetermined scale reading in response to the injection of the standard sample 6. The apparatus of claim 5 wherein said autoranging circuit includes a multiplying circuit, said light detecting means being coupled to the input of said multiplying circuit; a comparing circuit for comparing a reference signal representing the predetermined scale reading of the recorder with the output signal of said multiplying circuit; said comparing circuit being coupled to said multiplying circuit to control said multipying circuit in response to the difference between said reference signal and the signal from said multiplier that corresponds to the signal produced by the standard sample.

7. The apparatus of claim 3 wherein the hydrocarbon gas is stored as a liquid hydrocarbon.

8. A method for determining the quantity of a metallic element in a sample wherein burning of the sample produces background light having a definite slope, said method comprising the steps of:

burning the sample;

intercepting the light emitted by said burning;

producing a spectrum of the light intercepted;

scanning said spectrum with an exit slit vibrated completely across the spectral line of interest to include a first portion of the spectrum that does not include the spectral line of interest, a second portion of the spectrum that does include the spectral line of interest and a third portion of the spectrum that does not include the spectral line of interest;

generating a reference signal of twice the scanning frequency of said exit slit;

separating the signal produced by scanning the spectrum and the signal of twice the scanning frequency of the exit slit to reject background light which is not constant but which has a definite slope thereby passing only the signal due to the spectral line of interest; and

recording the signal due to the spectral line of interest.

9. The method of claim 8 further including the step of periodically substituting a sample stream having a known concentration of the metallic element of interest for the process stream and recording the resulting signal on a scale related to the known concentration. 

1. An apparatus for detecting the quantity of a metallic element in a process stream, said apparatus comprising: a burner assembly including means for supplying a fuel gas, air and a sample of the process stream thereto; a monochromator disposed to intercept the light emitted by the flame, said monochromator having a narrow fixed entrance opening and means for producing a spectrum of the light passing through the entrance opening; a narrow exit slit opening disposed to pass a narrow band of light from said spectrum, a drive means coupled to said exit slit opening and disposed to vibrate said exit opening at a predetermined frequency; light detecting means comprising means for detecting the modulation of the light from said vibrating exit slit at the fundamental frequency of vibration of said slit and at twice the fundamental frequency of said slit and converting a signal representing twice the fundamental frequency to a related electrical signal; and recording means coupled to said light detecting means to record said electrical signal in relation to time.
 2. The apparatus of claim 1, and in addition an autoranging means for adjusting the range of the recording means in response to the introduction into the burner of a sample having known amounts of metallic elements.
 3. The apparatus of claim 1 wherein said fuel gas is a hydrocarbon.
 4. The apparatus of claim 1 wherein said exit slit opening comprises a vane member mounted on said drive means for movement in a plane disposed at right angles to the beam of light from the spectrum producing means, said vane having a narrow elongated slit formed therein for passing said narrow band of light.
 5. The apparatus of claim 2 wherein said autoranging means comprises circuit means for injecting into the burner a standard sample having a known composition and a means for adjusting the recorder to a predetermined scale reading in response to the injection of the standard sample.
 6. The apparatus of claim 5 wherein said autoranging circuit includes a multiplying circuit, said light detecting means being coupled to the input of said multiplying circuit; a comparing circuit for comparing a reference signal representing the predetermined scale reading of the recorder witH the output signal of said multiplying circuit; said comparing circuit being coupled to said multiplying circuit to control said multipying circuit in response to the difference between said reference signal and the signal from said multiplier that corresponds to the signal produced by the standard sample.
 7. The apparatus of claim 3 wherein the hydrocarbon gas is stored as a liquid hydrocarbon.
 8. A method for determining the quantity of a metallic element in a sample wherein burning of the sample produces background light having a definite slope, said method comprising the steps of: burning the sample; intercepting the light emitted by said burning; producing a spectrum of the light intercepted; scanning said spectrum with an exit slit vibrated completely across the spectral line of interest to include a first portion of the spectrum that does not include the spectral line of interest, a second portion of the spectrum that does include the spectral line of interest and a third portion of the spectrum that does not include the spectral line of interest; generating a reference signal of twice the scanning frequency of said exit slit; separating the signal produced by scanning the spectrum and the signal of twice the scanning frequency of the exit slit to reject background light which is not constant but which has a definite slope thereby passing only the signal due to the spectral line of interest; and recording the signal due to the spectral line of interest.
 9. The method of claim 8 further including the step of periodically substituting a sample stream having a known concentration of the metallic element of interest for the process stream and recording the resulting signal on a scale related to the known concentration. 